I buy a product from West System, for thickening epoxy resins
used in boat building, that West System calls 407 'collodial silica'

It looks like Frog's Fanny to me. Other fiberglass supply businesses
sell similar products they often refer to as "cabosil"
For ten bucks or so you get about quart.

For 18 bucks I get nearly a gallon sized container.
This may or may not be the specially treated (something about
hydrophobia?)
cabosil others have said was required.

But it works just fine for drying flies. I've been using it for years.